The present invention relates to a method and apparatus for removing water from a fiber web that has been formed by one or more endless wire belts, particularly in a paper manufacturing machine, wherein the web is guided in a (generally) horizontal direction of travel of the machine. Water is removed by conducting the web through several press nips in succession, and an endless, water absorbing porous belt, preferably a so-called "felt belt" or "wet felt", travels with the web through at least one of the press nips. The web is conducted from press nip to press nip such that it never need travel free and is always supported by a wire belt, a felt belt or the outer surface of a roll.
Literature showing the prior art:
D1: U.S. Pat. No. 4,285,766 PA1 D2: Federal Republic of Germany 273783, FIG. 2 PA1 D3: Federal Republic of Germany 3425077, which is equivalent to U.S. Pat. No. 4,662,992 PA1 D4: Federal Republic of Germany 3808293, which is equivalent to U.S. Pat. No. 4,923,570 PA1 D5: U.S. Pat. No. 4,556,451 PA1 D6: Federal Republic of Germany 4026021 (FIG. 3), which is equivalent to U.S. Pat. No. 5,178,732
In many modern paper manufacturing machines, water is removed from the web by the method described in the first reference D1 or by similar methods. In such a method, the web is first conducted between the wire belt on which the web is formed and a first felt belt which is supported by a take-up suction roll, through a first press nip. Thereafter, the web travels further together with the first felt belt. The first felt belt, which is supported by a suction press roll, then conducts the web through two additional press nips, and the web is conducted by the smooth outer surface of a press roll through at least one additional press nip. This known method has proven satisfactory. However, it disadvantageously requires a large amount of space and there are high expenses for the purchase and operation of such a press section, particularly with regard to the suction rolls.
The second proposal described in reference D2 dates back to the initial days of the manufacture of paper by machine. The paper web is transferred in the first press nip from the wire belt to the smooth outer surface of a first press roll and is then conducted by the first press roll through additional press nips, namely through two felted and one non-felted press nips. After the latter nip, the web is conducted by a further smooth press roll into a final felted press nip. In this arrangement, the web passes through the last press nip in a direction opposite the direction of travel of the machine. Therefore it must be first conducted further, downstream of the press nip, by the last felt belt. Rewetting of the web by the felt belt results, causing inadequate dryness of the web in practice. The very large diameter of the first press roll as compared with the diameter of the other press rolls is also unfavorable. For these and other reasons, the above described known apparatus is not suitable for use in modern paper manufacturing machines.
In the third reference D3, the web is again removed from a wire belt by a smooth press roll and is conducted through a felted press nip. The latter felted press nip is lengthened in the direction of travel of the web by being a so-called shoe-press roll which includes a flexible, revolving press jacket that passes over a press shoe. The press jacket comes into contact with the felt belt in the press nip. Another difference in principle between above references D2 and D3 is that in D2, as is generally customary, the wire belt contacts the lower side of the web, upstream of and within the first press nip, while in D3, the wire belt contacts the top side of the web. In D3, the web is formed in a twin-wire zone and after that zone, the web continues to move in an upward direction together with the upper wire. Upstream of the first press nip, a steam blow box and suction boxes are provided therein for the removal of water from the web. This has the disadvantage that the press rolls must be arranged at a relatively great height above the twin-wire zone and above the following drying section. This known apparatus is therefore not advisable, in part because of the necessity of expensive and tall support frames, with the danger of mechanical vibrations, in further part because of the hot air rising from the inlet into the drying section, and also because the operation and accessibility during operation are made difficult by all of these circumstances, in particular during the work that is periodically necessary for replacing rolls and wire and felt belts. Another disadvantage of the method described in reference D3 is that the removal of water from the web is effected only toward one side of the web. This provides a danger that the final web of paper will have dissimilar properties on its two sides, causing a two sided nature of the paper web. The guidance of the paper web through the following drying section, namely according to FIGS. 1 and 3, wherein the upper side of the web comes into contact with bottom drying cylinders, is also unfavorable. This makes removal of any possible broke very difficult.
In above reference D4, various wet press arrangements are described having a shoe press roll with a flexible rotating press jacket having a smooth outer jacket surface which comes into direct contact with the paper web. On a stationary support member, a radially movable press shoe is supported. The shoe has a concave slide surface. Alternately, several press shoes of this type are provided. In addition, rigid ledges with convex slide surfaces are provided.
The wet press arrangement described in above reference D5 comprises inter alia three press nips arranged one after the other along the web path, namely a felted press nip, an unfelted press nip, and a final felted press nip. However, no shoe-press unit is present, i.e. all of the press rolls are ordinary tubular rolls. Therefore, there is insufficient water removal capacity in this press.
In FIG. 3 of above reference D6, two shoe press rolls together form the feltless press nip. It is difficult to assure a dependable transfer of the paper web from one press jacket to the next for certain types of paper.